Laser welders are extensively used to weld sheets of metal together. In some applications, the sheets of metal serve as walls confining fluids under pressure, and hence the welds must be impervious to leakage. If structures incorporating the welded sheets are small and made in small numbers, the structures may be visually inspected or tested under pressure when completed and any faults in the welds repaired as necessary. However, when the amount of welding is extensive and the final structures not easily tested, it is very advantageous to detect defective welds as they are made and to mark the location of the defects. Doing so greatly facilitates their later repair.
Numerous conditions can cause faulty or defective welds. Foremost among them is interruption of normal laser operation. Further, if the sheet metal being welded is contaminated or if the sheets being joined are not clamped tightly together, the welds will be defective even though the laser itself is operating properly. The present invention makes it possible to detect faulty welds as they occur regardless of their cause. This is accomplished by noting the acoustic signals that are characteristically generated by the operation of the laser. By comparing those signals to a signal pattern characteristic of normal welding operations, the fact that a faulty weld has been made can be detected, and means can be activated to mark the location of the defect.